Anyone heard of eggbound prevention that makes shells flexible?

I have lost 3 button quail hens to being eggbound. They have the largest egg to bird size and so it is not uncommon. Someone told me their grandfather used to add something to the chickenfeed to make the shells more flexible and pass easier. Anyone heard of such a thing? I give my chickens their own shells and oystershell and have not had a problem in the 3 years I have had them. The quail eggshells are hard and thick and I give them calcium in the same above forms as well as in the gamebird feed. It is apparently simply the size of the egg that causes the problem so if it were able to stretch a little might it help? Thanks for any tips- Suzanna

I am not that familiar with buttons, but we use black oil sunflower seed for chickens (unhauled) to help keep the reproductive tract lubricated. I wonder if you could grind them up for the buttons and offer them that way? Hopefully someone will come with more experience in this.